Intel Pentium 4 1.8A socket: Socket 478

What the Pentium 4 1.8A is

The Intel Pentium 4 1.8A is an 1800 MHz single-core desktop processor from July 2001. It runs one NetBurst Willamette core at 1800 MHz with no Hyper-Threading, so 1 core gives 1 thread. Cache is 256 KB of L2 on a 180 nm process with 42 million transistors. Power is rated at 69 W in a Socket 478 FC-PGA2 package with part number SL63X on a 400 MT/s front side bus with an 18x multiplier at 1.75 V.

The A suffix on this Willamette part predates the Northwood 1.8A of January 2002 that reused the name with 512 KB of cache on 130 nm, which is why board lists must separate the 69 W Willamette 1.8A from the 49.6 W Northwood 1.8A. Intel sold the Willamette 1.8A in July 2001 as a Socket 478 bridge part. The chip supports MMX, SSE and SSE2.

An 1800 MHz mark with a full 400 MT/s bus suits mid-2001 Socket 478 towers with SDRAM or DDR paths. The 18x ratio is a clean multiple of the base clock. That is why the Willamette 1.8A shipped in mainstream Socket 478 desktops before Northwood arrived.

Socket and motherboard compatibility

The 1.8A fits Socket 478 mainstream desktops with Willamette support. It needs a 478-pin board with a 400 MT/s bus and a BIOS that lists the Willamette Pentium 4 1.8A at 69 W. The Socket vs chipset: what decides whether a CPU works guide covers pairing rules.

Mainstream DDR towers are the natural match. Boards listing SL63X accept the chip with a 69 W cooler. Northwood 1.8A strings at 49.6 W need their own BIOS entries, so the sSpec line decides which 1.8A boots.

Memory, PCIe and cooling

Memory is SDRAM, DDR and RDRAM through the board chipset by board generation, matched to the board slots. The CPU has no memory controller. Mainstream 845 boards commonly run DDR200 pairs for 1800 MHz Willamette parts.

A 400 MT/s front side bus ties the CPU to the chipset, while AGP and PCI hang off the chipset. No PCIe lanes originate on the die. Cooling uses the Socket 478 bracket with a cooler rated for 69 W.

Upgrade paths

On the same socket, a move to a Northwood 1.8A keeps the 1800 MHz mark while doubling cache to 512 KB at 49.6 W when the BIOS lists Northwood strings. The swap cuts power while adding cache, so thermals improve. A Pentium 4 1.80 such as Intel Pentium 4 1.80 is the non-A Willamette 1.8 at 67 W on the same bus.

A move to Pentium 4 2.0 Willamette adds 200 MHz when power tables allow. The Socket 478 page lists the Willamette range for planning.